A photoplethysmograph is an apparatus for diagnosing venous incompetence in the extremities, the presence of arterial supply, and the obstruction of the carotid arteries. It is extremely sensitive to the relative amount of blood in a capillary bed and to arterial pulsation.
In general, a photoplethysmograph comprises a probe coupled to an adjustable gain AC and DC amplifier. The outputs of the amplifiers are coupled to a chart recorder for displaying signals detected by the probe.
In the probe there is provided a light source and a photosensor. Typically the light source comprises a light-emitting diode (LED). An LED is used since it produces light in a narrow spectral band and has a low power dissipation. Low power dissipation is essential since any heating of the skin against which the LED is placed produces a vasoactive response which will cause an erroneous measurement. The wave length of the radiation from the LED is typically in the range of 800-950 nanometers.
The photosensor may be a photocell, phototransistor or photodiode which is provided with an infrared filter to reduce its sensitivity to ambient light. A resistive sensor could also be used; however, it is considered to be inferior because its speed of response depends on ambient light level and is too slow to allow visualization of the dichrotic notch that is present in arterial pulsations. In addition, the sensitivity of a resistive sensor depends exponentially on the light level. In contrast, phototransistors and photodiodes are both quite linear and have a rapid response and are sensitive to the near-infrared.
In many cases, the diagnosis and treatment of medical conditions require numerous tests which extend over a long period of time. In such cases the diagnostician is frequently looking for relatively small changes in the medical condition. In order to do this, it is important that the medical instrumentation used by the diagnostician be accurate and repeatable--that is, nothing should change in the instrumentation between uses thereof which impairs the measurement of or otherwise masks small changes in a patient's medical condition which may have occurred since the last test. Alternatively, where changes in the medical instrumentation do occur, a means must be provided for calibrating such instrumentation to compensate for those changes.
In photoplethysmographic apparatus, changes in the instrumentation which can impair the measurement of small changes in medical condition from test to test or otherwise mask such changes are manifold. For example, in any given instrument, there may be, between tests, changes in the LED current used for driving the LED, a change in the LED brightness, a change in the phototransistor sensitivity, a change in the gain of the AC and DC amplifiers in the photoplethysmograph apparatus, gradual changes in the optics--such as face plate scratches or surface contamination of the sensing units--a change in the gain of a chart recorder, or changes in the frequency response of the photoplethysmograph amplifiers. Some of the above changes may be due to aging of the apparatus. Others may be due to ambient temperature. Still others may be due to changes in the output of the power supply used in the apparatus. Moreover, the output of the apparatus may change when changing sensor modules, the amplifiers, or the chart recorders in any given system.
Heretofore, so far as is known, there has been no suitable means proposed for calibrating an otherwise conventional photoplethysmograph apparatus to compensate for the above described changes.